1. Field of the Invention
The present invention relates to a hydraulic jack and, more specifically, to a lifting mechanism for hydraulic jack, in which, when lifting a heavy load, high pressure in the inner tube forces the return fluid control pin to open the steel ball of the pressure regulating valve, for enabling the internal hydraulic fluid of the big piston to flow back to the cylinder and the internal hydraulic fluid of the small piston to be pumped into the inner tube, so as to lift the load with less effort.
2. Description of the Prior Art
Regular hydraulic jacks commonly use a piston pump to such and deliver a hydraulic fluid, causing the hydraulic fluid to move a lifting ramp so as to lift the load. A piston pump for this purpose has a single piston, which provides a constant pressure to deliver the hydraulic fluid when reciprocated. Therefore, the user wastes much effort to lift the lifting ramp to the bottom side of the load to be lifted. There is known a two-step hydraulic jack with a piston pump of double-piston design. This double-piston design enables the user to lift the load more efficiently. Before touching the load, the big and small pistons of the double-piston design piston pump are synchronously reciprocated to extend the lifting ramp quickly. After reaching the load, the small piston is reciprocated to pump the hydraulic fluid to force the lifting ramp outwards, and to further lift the load at a relatively lower speed.
According to conventional stepless hydraulic jacks of double-piston design, the maximum operating force is at the status when the lifting ramp receives 100% of the set pressure. Normally, the weight of the load is below 40% of the set pressure. Therefore, it is easy to lift the load of which the weight is below the set pressure. When the weight of the load is below 40% of the set pressure, the big and small pistons of the piston pump can be synchronously operated to extend the lifting ramp and to further lift the load with less effort. In a two-step lifting type hydraulic jack, the lifting ramp can be quickly extended out when receives no pressure. However, when the lifting ramp receives a pressure (for example, 10% of the set pressure), the internal hydraulic fluid of the big piston must be forced to open the steel ball of the pressure regulating valve, for enabling the internal hydraulic fluid of the big piston to flow back to the cylinder, leaving the internal hydraulic fluid of the small piston to be forced into the inner tube to extend out the lifting ramp and to further lift the load. In this case, the user must employ much effort to provide a lifting pressure equal to 100% of the set pressure plus 10% of the set pressure to overcome the pressure received by the lifting ramp. Therefore, the user must give employ much effort to lift the load. When a two-step lifting type hydraulic jack is used to lift the load of which the weight surpasses 40% of the set pressure, the user must employ much more effort to provide a lifting pressure equal to 100% of the set pressure plus 40% of the set pressure to overcome the pressure received by the lifting ramp. This lifting pressure demand is beyond the operable range of the hydraulic jack. In order to prevent the occurrence of this problem, rapid lifting mode can be operated only when the lifting ramp receives no pressure or a small amount of pressure.
The present invention has been accomplished under the circumstances in view. According to the present invention, when the lifting ramp receives a pressure over a predetermined value, the internal hydraulic fluid of the inner tube immediately forces a return fluid control pin to open the steel ball of the pressure regulating valve, for enabling the hydraulic fluid to flow back from the big piston of the piston pump to the casing. Therefore, less effort is needed to lift the load. The hydraulic jack lifting mechanism according to the present invention comprises a valve seat coupled to a cylinder and an inner tube in the cylinder, a piston pump, the piston having a big piston and a small piston respectively aimed at a high-pressure fluid hole and a low-pressure fluid hole in the valve seat, a return fluid control pin, a pressure regulating valve, an one-way ball valve unit, which controls one-way flowing direction of the hydraulic fluid to extend a lifting ramp out of the inner tube to lift the load, and a return fluid valve. When lifting a heavy load, high pressure in the inner tube forces the return fluid control pin to open the steel ball of the pressure regulating valve, for enabling the internal hydraulic fluid of the big piston to flow back to the cylinder and the internal hydraulic fluid of the small piston to be pumped into the inner tube, so as to lift the load with less effort.